Diamond buffer

The diamond follower was a natural development of the complementary emitter follower with diode biasing. Harris Corporation used it in the output stage of the HA-2600 monolithic operational amplifier. American press of the 1970s analyzed the HA-2600 in detail, but did not give its output stage a specific name The circuit remained uncommon, because early fabrication processes could not produce high-quality pnp transistors. The first dedicated diamond buffer circuit, the 30-MHz LH0002, was introduced by National Semiconductor in the late 1970s, and then described merely as "wide band, high current, unity gain buffer amplifier". By 1982, the LH0002 and similar discrete-transistor buffers were widely used in low-power (1 W or less) applications, particularly for video and instrumentation purposes. A circuit identifiable as the diamond buffer appears in the discrete power output stage of the Jordan Professional 440 Solid State IC Amp Head, a guitar amplifier from 1970. True high-speed integrated buffers became a reality only after the introduction of the silicon on insulator technology in the late 1980s, which led to the development of modern integrated current-feedback operational amplifiers (CFOA). The CFOA normally contains two diamond buffers. A "diamond transistor", in Burr-Brown language, denoted a diamond buffer with an added high-impedance current-output stage. The marketing terms were adopted by audio amplifiers designers, despite ambiguity with the diamond cubic crystal structure, the hypothetical diamond transistors, and the "diamond differential" topology marketed by Sansui around 1980. In English-language academic literature, the diamond buffer has also been called "mixed translinear cell II" or MTC-II, to differentiate from the plain "mixed translinear cell" (another name for a diode-biased emitter follower). Outside of the anglosphere, the classic German textbook by Tietze and Schenck discusses the circuit as one of many forms of biasing the emitter follower, without giving it a specific name. Russian-language authors ambiguously call it the "linear parallel amplifier" or merely "parallel amplifier". However, around 1982 == Operation ==

The circuit is symmetrical, and therefore it may be analyzed by examining only the upper (T1, T2) or the lower (T3, T4) half. In practice this is rarely necessary, and the offset is either left alone, or suppressed with negative feedback, or isolated from the load with a coupling capacitor. Clipping behaviour Ordinary two-stage ("doubles") or three-stage ("triples") emitter followers clip when the instant input voltage approaches either positive or negative supply rail. Clipping due to current starvation of the output stage is not a concern because the preceding stages are almost always able to deliver the required currents into the bases of the output transistors, even when their beta droops at very high output currents. The diamond follower behaves differently, because the base currents of the output transistors (T2, T4) are limited by constant current sources (Ie1, Ie2). In absolute terms, the manufacturer of the LH002 specified open-loop THD of 0.1% at 5 V RMS output into a 50 Ohm load at ±12 V supply voltage (class AB). Designers of class AB zero-feedback hybrid audio power amplifiers employing the unmodified four-transistor output stage claimed THD of 0.1% at 3 kHz and 0.25% at 20 kHz. Slew rate The slew rate (SR) of a simple diamond buffer is limited by Ie1 at SR=Ie1/Cint1, where the internal capacitance Cint1 is the total capacitance "seen" by the current source Ie1 at the common node of the base of T2 and the emitter of T1 (or, in case of Ie2, at the common node of the base of T4 and the emitter of T3). For example, a 0.5 mA current source loaded into 10 pF node capacitance has SR of 50 V/μs. Negative and positive slew rates may be markedly asymmetrical. The power bandwidth for peak output voltage Vp is limited by the least of these two slew rates at Fmax=SR/(2πVp). If the slew rate of the input signal exceeds the SR of the diamond buffer, the circuit may experience thermal runaway - a potentially destructive scenario when both T2 and T4 are conducting. The frequency that triggers thermal runaway for a given peak input voltage Vp is defined by the same formula as the power bandwidth, Fmax=SR/(2πVp). == High output current derivatives ==

Clipping caused by current starvation of the input transistors is particularly pronounced in circuits with simple resistive "current sources", and in circuits where T2, T4 operate at fairly high current densities and thus exhibit strong beta droop. The use of electronic constant current sources and large-area, sustained-beta output transistors delays the onset of clipping, but does not change the pattern. An increase in idle currents allows a proportional increase in current handling, however, high idle currents invariably increase power consumption and heatsinking requirements. For example, each channel of a commercial Dartzeel 108 audio amplifier delivers up to 160 W into a 4 Ohm load from a simple unmodified diamond, at a cost of dissipating around 40 W idle power and weighing 15 kg. A hybrid follower adds two simple emitter followers T5, T6 to the input stage. When the latter switches off, one of the added transistor provides the required base current to the output stage. This circuit, too, suffers from crossover distortion. Slew rate at small input currents remains unchanged, but increases in a nonlinear, intermittent fashion when T5 or T6 engage. A follower built with small-signal transistors and drawing 1 mA idle current can easily attain high-level slew rate of 1000 V/μs. However, when input voltage decreases, slew rate abruptly drops to its much lower (low-level) natural value. In a quasi-Darlington configuration, the added transistors T5, T6 sense the currents flowing from the current sources into the bases of output transistors T2, T4 and inject additional currents into their bases, thus preventing starvation of T1, T3. The arrangement is not a true Darlington circuit because T5, T6 engage only temporarily, at very high output currents. True Darlington outputs had also been proposed, albeit limited to class B operation. Finally, the diamond buffer does not have to drive the load directly. The additional high-current transistors can be inserted between the buffer and the load, providing the required current reserve. In a "diamond buffer Triple" configuration the added transistors form a conventional emitter follower. The drawback is that the circuit requires its own bias spreader for thermal regulation. Emitter resistors in the output stage are not necessary for thermal stability, but are critical for minimizing crossover distortion. Least distortion is attained when the voltage drop across each emitter resistor at idle current equals the thermal voltage (26 mV at 300 K). A simpler solution is to replace the output devices with two Sziklai pairs, which do not need the bias spreader and do not introduce significant thermal drift into the basic diamond structure. Transistor T1-T4 must be thermally coupled together, but T5 and T6 should be outside this thermal feedback loop. The idle currents of T3, T4 are regulated with purely electrical local feedback via emitter resistors Re1, Re2. Voltage across each resistor, again, should be set to 26 mV. == Notes ==